This invention relates to a process for preparing rubber products which comprises crosslinking rubber in the presence of oxygen such as in air or the like with the use of an organic peroxide as a crosslinking agent.
Crosslinked rubber products prepared with the use of an organic peroxide as the crosslinking agent of rubber have excellent properties such as high heat aging resistance, low compression set, decreased staining to metal or coated metal sheet, easy production of colored rubber products which give less color change during crosslinking or after usages for a long period in comparison with sulfur crosslinked products, and have hitherto been used for the production of parts for automobiles, industrial parts, building rubber materials, electrical insulating materials and the like. However, in the process for producing crosslinked rubber products with the use of organic peroxides, processes to accomplish crosslinking under air free atmosphere, e.g. within molds of heat processes or injection molding machines or in fused salt baths (continuous crosslinking method named LCM) or the like, have been employed. Heating apparatuses working in the presence of air such as hot-air heaters, high-frequency heaters or fluidized bed heaters have not been employed. The explanation is found in the fact that when crosslinking with an organic peroxide is carried out by using a heater in the presence of air, the surface of the resulting crosslinked rubber product will become extensively tacky and thus the product will lose its commercial value. Further, when crosslinking is carried out by using a steam vulcanizing autoclave, it is necessary to replace air in the autoclave with steam of the heating source and to remove the air, which causes restriction for practical operations, especially for batch-wise autoclave crosslinking.
Furthermore, as for the field of rubber foams (rubber sponges), the rubber foams prepared by the use of sulfur as a crosslinking agent are extensively used in various sponge products for automobile, building and the like. However, it has been desired to improve the properties of these foams such as heat aging resistance, compression set, staining to coated metal sheet, discoloration or color changes of colored sponges and the like.
These problems needing improvements are caused by the use of sulfur as the crosslinking agent and thus they can be solved by the use of an organic peroxide in place of sulfur as the crosslinking agent.
However, most of the rubber foams are produced in the presence of oxygen such as air. Above all, for continuous production processes in which crosslinking and foaming are carried out swiftly in a short time, the use of a hot-air heater, a high frequency heater or the like generally prevails. These rubber-foam products crosslinked with an organic peroxide in the presence of oxygen will become extensively tacky on their surfaces just link the aforementioned non-foamed rubber products. Thus, these products not only have no commercial value but also adhere to conveyer belts or rolls during their production processes and interfere with the production of their articles.
As a method of preventing the tacky surfaces of the crosslinked rubber products, there are disclosed methods in which the surface of a non-crosslinked formed article is preliminarily treated with a special agent, and the treated article is then subjected to crosslinking (U.S. Pat. No. 4,334,043 and EP-A No. 73,037). These methods require the removal of the surface treating agent. However, particularly in products having complicated shapes, it is difficult to wash off the agent completely, and the washing procedure will cause another problem that of treating the waste water. Therefore, such methods are not available for practical use.
Accordingly, it has been hitherto recognized that crosslinking with an organic peroxide is not done in the presence of oxygen. Such a recognition has been a matter of common knowledge in the art of manufacturing rubber articles.
This invention is to provide a process for producing novel rubber products crosslinked by organic peroxides in the presence of oxygen which provide a tack-free surface without the use of surface treating agents.